The sight of a clownfish wriggling through the stinging tentacles of its anemone is a familiar and seemingly well-understood one to most people—the stinging anemone provides a protective home for the clownfish who is immune to such stings, and in turn the clownfish chases away any polyp-eating sunfish eyeing the anemone's tentacles for a meal. But recent research has shown that all that clownfish wriggling significantly helps to oxygenate the anemone at night, when oxygen levels in the water are low.

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"Anemones are not only sedentary but also lack the ability to self modulate the water flow over their tissues," says Joseph Szczebak lead author of the paper in the Journal of Experimental Biology. "As such, they are particularly prone to succumbing to low oxygen conditions. By aerating their host anemones, anemonefish [i.e. clownfish] ensure their hosts have a constant supply of oxygen-rich water."

This behavior is a prime example of the symbiotic relationships that are a hallmark of life in the coral reef ecosystem the clownfish and anemone call home, in which one species helps another satisfy a need it would have trouble fulfilling alone in exchange for help of a similar kind. The clownfish and anemone are almost quite literally scratching each others' back, albeit with perhaps a softer, more snuggly touch that leads to easier breathing.

For their experiment, Szczebak and his Auburn University colleagues observed the nocturnal behavior of the clownfish—also known as the anemonefish—in several ways: when it was with the anemone, when it was completely separated from the anemone, and when it was only separated from the anemone by a mesh screen, allowing the animals to be aware of the others' presence.

There were two mesh variations (pictured below)—including one with the water flow passing by the anemone first, and the other with the flow passing by the clownfish first—which were included to see whether simply the sight or smell of each other had any effect on behavior and oxygen uptake. "Sea anemone chemical compounds" have been observed to "directly influence the recruitment and recognition behaviors of anemonefishes toward host sea anemones," noted Szczebak to mongabay.com of previous scientific literature, but "the extent to which sea anemone chemical cues influence anemonefish behavior at night has yet to be discerned."

The researchers found that though the smell of the anemone did result in more "switching" behavior from the clownfish, it was only when the two were together and able to make physical contact—the clownfish "wedging" deep into the anemone, "switching" directions abruptly, and "fanning" its fins—that oxygen uptake significantly increased. In addition, this behavior was far less frequent when the clownfish was not with the anemone.